The present invention generally relates to hearing aids and more particularly relates to a hearing aid that improves the hearing experience and to a method for fitting such a hearing aid to a user of the hearing aid.
Hearing aids typically consist of a microphone, a signal processor, and a loudspeaker (sometimes called a “receiver” or “output transducer”). The loudspeaker directs sound picked up by the microphone and processed by the signal processor into the ear canal and is held in place using an earpiece that fits in the ear canal. The earpiece changes the acoustic environment of the ear drum and therefore the perception of sound in the same way that a mute inserted into the bell of a trumpet changes its timbre. In general, an earpiece will attenuate at least some frequencies from the sound incident on the ear drum. This phenomenon is often called the “insertion effect”.
An earpiece that does not completely block the ear canal is said to be an open earpiece. Usually this style of earpiece is accomplished using several cross-sectional holes in a thin and light-weight earpiece. As a result, there is an open-air path for sound to travel to the ear drum, which allows some frequencies, usually lower ones, to reach the ear drum with no attenuation. Higher frequencies tend to be blocked by the earpiece and are therefore moderately attenuated. Overall, in instances where an open-ear earpiece is used, the insertion effect affects only a portion of the audible bandwidth.
An earpiece that completely blocks the ear canal, usually with a heavy plastic or silicon earpiece mold, is said to be a closed earpiece. In a closed earpiece, there is no air-conduction path for sound to travel directly to the ear drum and thus all sound frequencies are severely attenuated. This attenuation allows the hearing aid to provide more gain before feeding back to the input microphone. However, blocking the entirety of the external portion of the ear canal causes the user's own voice to resonate in the closed cavity, amplifying the low frequency harmonics. This “head in a barrel” effect, sometimes called the occlusion effect, can be mitigated in part by introducing a vent through the earmold. The larger the diameter of the vent, the more the occlusion effect is reduced. However, this vent has the disadvantage of reducing the earmold's attenuation at lower frequencies and thus the additional gain that a closed-ear earpiece can provide at those frequencies.
The natural resonance of the ear depends on ear shape, which varies considerably between individuals. As a result, the exact insertion effect caused by an earpiece differs from person to person. This variability in the insertion effect is why earpieces are generally custom molded for an individual's unique ear shape and is why a fitting process is needed to compensate for an individual's unique insertion effect.
A typical hearing aid works by providing frequency dependent amplification, or gain. More recent developments have provided improvements to typical hearing aids by introducing gain that not only depends on frequency but also on level. In either case the gain needs to be tuned to match the user's particular hearing loss for each ear and this is done in a process referred to as “fitting.”
Many known fitting processes have been developed by hearing aid manufacturers and academic research groups. These fitting processes are generally oriented toward a multiband architecture where there is minimal frequency overlap between bands and generally ignore the potential artifacts that come from phase distortion and combination effects.
One such artifact results from the latency of the hearing aid, that is, the time delay between when a sound is sensed at the microphone and when it is converted to an acoustical sound wave at the hearing aid's loudspeaker. For modern digital hearing aids, the latency is between 3-7 milliseconds. Older analog hearing aids had a latency around 1-2 milliseconds. When both the incident sounds perceived by the ear drum and amplified sounds produced by the hearing aid are similar in sound level, any non-zero latency of the hearing aid causes comb filtering, a form of spectral distortion. Comb filtering is characterized by a series of regularly spaced spectral peaks and dips in the sound pressure at the ear drum. For longer latencies, the first dip is at a lower frequency and hence a larger portion of the frequency spectrum is affected. Shorter latencies produce less extensive comb filtering. The human ear is very sensitive to this kind of artifact. Latencies shorter than 8 milliseconds are perceived as tone coloration, while longer latencies can be perceived as echoing, beating, or tone coloration, depending on the relative loudness of the delayed sound.
Another recombination artifact arises from phase distortion in the amplified sound. This also produces a structure of spectral dips and peaks. Frequencies that are 180 degrees out of phase recombine destructively, creating a dip, while frequencies that are in phase add constructively, creating a peak. Since phase distortions are often spread non-uniformly over the frequency spectrum, this kind of artifact potentially affects less of the spectrum than latency artifacts. Phase distortion caused by summation is greatest when the direct sound is as loud as the amplified sound. The source of phase distortion can be any component in the signal path including the microphone, signal processing components, or the loudspeaker. Recent evidence suggests that phase is used by humans for many hearing tasks, including source localization, speech encoding, and detection of phase modulation.
The present invention provides a means for a hearing aid and a method of fitting a hearing aid to a user that both avoids the coloration that results from an altered ear canal resonance (i.e., the insertion effect) as well as the artifacts caused by level-dependent gain. The hearing aid with either open or closed earpieces, when fitted in accordance with the invention, can provide the user with a much more “natural” or “transparent” perceived sound. The invention provides an earpiece that is transparent after it has been fitted. By providing transparency, the user's hearing loss can be more effectively corrected.